Periodontal disease is a disease that affects a portion around a tooth, that is, the gums (gingiva), the alveolar bone, the periodontal ligament, and the cementum portion below the gums. However, in general, periodontal disease indicates inflammatory disease affecting a portion around a tooth, which makes up 90% or more of the patients. Accordingly, a common periodontal disease is caused by bacteria such as Porphyromonas gingivalis. The present invention also focuses on a common periodontal disease. Hereinafter, a common periodontal disease will be referred to as “periodontal disease”, while other periodontal diseases will be referred to as “special periodontal disease”.
Hereinafter, the stages of periodontal disease will be described. Although a mandibular tooth, whose root apex points downward is taken as an example, the same applies to a maxillary tooth. FIG. 1 is a cross-sectional view schematically showing a sound tooth, and FIG. 2 is a cross-sectional view schematically showing a tooth with periodontal disease. As shown in FIG. 1, a sound tooth 30 has its root 31 supported by a periodontal ligament 32a, an alveolar bone 35, and gums (gingiva) 32. A space between the root 31 and the gums 32 is called a periodontal pocket 45 (hereinafter, simply referred to as “pocket” or “gingival sulcus”).
If affected by periodontal disease, the gums (or gingiva) 32 becomes red and swollen, the periodontal ligament 32a is breached, and as shown in FIG. 2, blood and pus 48 ooze from the periodontal pocket 45 to start melting the alveolar bone 35. A depth of the pocket 45 from the gums top 46 to a pocket bottom 47 is about 0 to 1 mm in the case of a sound tooth, while the depth is deepened with the progress of periodontal disease in the case of a tooth 30 affected by periodontal disease.
Before starting the treatment of periodontal disease, the tooth is tested to determine to what extent the current symptom has progressed first, and then, how the symptom will develop needs to be determined. Various testing methods have been developed so far. The currently used major testing methods include those by probing, X-ray, and bacteria.
In the probing test method, a probe (a needle for test) is inserted into the pocket 45, and the state of the symptom is found out while measuring the depth of the pocket 45. This is the simplest method of the above, and can be applied to the symptoms at any stage. The method includes one-point method of measuring at the deepest portion, four-point method of measuring at mesial and distal sides and buccal and lingual sides, and six-point method of measuring at two more points in addition to the foregoing four points.
In the testing method using X-ray, the bone level (indicating the level of the crest of the alveolar bone supporting the tooth between the root apex and the cement-enamel boundary) and the outline of the alveolar bone 35 are grasped from a radiograph (a dental radiograph or a panoramic radiograph) of the tooth 30 and the periodontal tissue.
In the testing method using a bacteriological examination, pathogenic bacteria of periodontal inflammation is examined to determine activity and progressiveness of periodontosis. That is, it is considered that different bacteria have different degrees of pathogenicity, and therefore the activity can be determined by identifying the bacteria.
Physiological mobility of a sound tooth, which is usually within a range of 0.2 mm, is a total deformation of the periodontal soft tissue (e.g., gingiva and the periodontal ligament) and the alveolar bone. The degree of mobility, which increases with the progress of periodontal disease, is increased either by an inflammatory change in the periodontal tissue (refer to FIG. 3) or by reduction in bone level (refer to FIG. 4). In the former case, the mobility can be improved by removing the cause of the inflammation, while in the latter case, the mobility is hardly lessened unless the periodontal tissue is renewed. In general, a plurality of factors comprehensively cause a tooth to move. Accordingly, it is important to discriminate between the mobility that can be lessened by periodontal tissue treatment (the mobility caused by inflammation) and the mobility that cannot be lessened (the mobility caused by quantitative decrease of the supporting tissue) (refer to Non Patent Literature 1).
By the way, the degree of the bone level reduction considerably varies among the mesial and distal sides and buccal and lingual sides of the same tooth. Observation of the convex and concave shapes of the bone level is a significant observation that helps selecting a remedy or predicting whether the periodontal ligament can be renewed. Bone level reductions progressing uniformly within the oral cavity are called horizontal bone defects, and those progressing quickly at a particular portion are called vertical bone defects. Vertical bone defects are further separated into three-wall, two-wall, and one-wall vertical bone defects by the number of side bone walls surrounding an exposed root surface at bone defect portion.
In treatment of a root surface, which is exposed where the bone has a vertical defect, dentists sometimes cannot control instruments sufficiently due to surrounding bone walls. In the case of vertical bone defects, the periodontal ligament remains not only at a portion at and close to the root apex of the tooth having the defect but also at the sides in a manner close to each other. Accordingly, it is considered that the periodontal ligament is easily renewed at the root surface exposed inside the defect. In particular, renewal of the periodontal ligament can be expected with a high probability at the portion of three-wall bone defect. The renewal can be expected also in the case of two-wall bone defect, if the width of the defect is narrow. However, renewal is considered to be difficult in the case of one-wall bone defect, in which the pocket is likely to remain, and thus the remaining bone wall is often removed by an osteoplasty.
Thus, it is important to correctly grasp the convex-concave shape of the alveolar bone for it affects the treatment of periodontal disease. Though the bone level can be grasped from a radiograph as described above, it is difficult to completely understand the three-dimensional convex-concave shape of the bone level because in a radiograph a tooth and the periodontal tissue is drawn by being projected onto a two-dimensional plane. Accordingly, an approach is adopted clinically in which the three-dimensional state of the bone level is inferred from, for example, values obtained by probing test. However, it is pointed out that probing measurement values themselves have a reproducibility problem. That is to say, it has been hard for dentists to correctly grasp the convex-concave shape of the bone level.
In recent years, more and more dental clinics have adopted computer tomographic equipment (dental X-ray CT scanner) tailored for dental use. Using a dental X-ray CT scanner, a precise three-dimensional image of a tooth and a jawbone can be obtained. Thus, a dental X-ray CT scanner is now essential for safely performing sophisticated dentistry such as implant dentistry.
A three-dimensional image obtained from a three-dimensional space of the living body contains not only image data of a surface of a three-dimensional object but also image data of a point inside the object, so the three-dimensional image is particularly called “volume data”. Medical diagnostic imaging apparatus for generating volume data is not limited to a dental X-ray CT scanner and includes various devices such as a medical X-ray CT scanner, a three-dimensional ultrasonic apparatus, a nuclear magnetic resonance apparatus, and a positron emission tomography apparatus.
Although volume data is image data of a point inside a three-dimensional space, an image display apparatus (monitor) for observing the data has a two-dimensional display surface. As a result, in order to appropriately draw the structure of a three-dimensional object to be observed onto the monitor, the volume data needs to be processed with image processing techniques into a two-dimensional image. Examples of a process of converting volume data into a two-dimensional image typically include multi-planar reconstruction, maximum intensity projection, shaded surface display, and volume rendering, which are known techniques (refer to Non Patent Literature 2).
Basic multi-planar reconstruction used for diagnostic imaging involves three image cross-sections: an axial cross-section obtained by horizontally cutting the body axis (or principal axis) which is set in a vertical direction; a coronal cross-section obtained by cutting an object widthwise in the left/right direction with respect to the body axis; and a sagittal cross-section obtained by cutting an object lengthwise in the front/rear direction with respect to the body axis. Hereinafter, the axial cross-section, the coronal cross-section, and the sagittal cross-section will be respectively referred to as an “A cross-section”, a “C cross-section”, and an “S cross-section” for the sake of simplicity.
It is, needless to say, important to manipulate the volume data for doctors to easily make a diagnosis, and a technique is disclosed in which displaying method has been improved to the shape of an object to be observed. For example, a technique has been described in which a medical image of a tube-like part such as the esophagus is converted geometrically and output to display means as a developed view for easily making a diagnosis on the state of the inner surface (refer to Patent Literature 1). Any point of the A, C, and S cross-sections can be displayed from volume data by a known technique, and navigation technique has also been described which helps understanding the correspondences among the cross-sections (refer to Patent Literature 2). According to Paten Literature 2, using both a diagnosis object image and a standard template, when a user specifies any spatial coordinate, A, C, and S cross-sections corresponding to the position of the coordinate are displayed, and also the correspondences among the cross-sections are shown as an intersection of a cross line.
Volume data obtained by photographing the inside of the oral cavity with a dental X-ray CT scanner contains detailed morphology information on a tooth and the surrounding tissue (e.g., the alveolar bone). The convex-concave shape of the alveolar bone, if understood from the volume data, can be used for setting up a treatment program for periodontal disease. In the dental and dental surgery fields, a technique for determining implantation site of implants or performing an orthodontic treatment has been described in which, if an anatomical feature point (landmark) is placed according to an arch shape of the row of teeth, the position of a cut section is determined based on the landmark, thereby showing the cut section (refer to Patent Literature 3). There have been no techniques disclosed, however, of manipulating volume data for easily making a diagnosis on the convex-concave shape of the alveolar bone.